Semiconductor chips or dies typically are manufactured from a semiconductor material such as silicon, germanium, or gallium/arsenide. An integrated circuit or other active feature(s) is incorporated in the die adjacent one surface, often referred to as the “active surface,” of the die. The active surface typically also includes input and output terminals to facilitate electrical connection of the die with another microelectronic component.
Since semiconductor dies can be degraded by exposure to moisture and other chemical attack, most dies are encapsulated in a package that protects the dies from the surrounding environment. The packages typically include leads or other connection points that allow the encapsulated die to be electrically coupled to another electronic component, e.g., a printed circuit board. One common package design, referred to as a board-on-chip (BOC) package, includes a semiconductor die attached to a small circuit board, e.g., via a die attach adhesive. Some or all of the terminals of the semiconductor die then may be electrically be connected to a first set of contacts of the board, e.g., by wire bonding. The connected board and die may then be encapsulated in a mold compound to complete the packaged microelectronic component assembly. A second set of contacts carried on an outer surface of the board remain exposed; these exposed contacts are electrically connected to the first contacts, allowing the features of the semiconductor die to be electrically accessed.
FIG. 1 schematically illustrates a conventional packaged microelectronic component assembly 10. This microelectronic component assembly 10 includes a semiconductor die 20 having an front surface 22, which bears an array of terminals 24, and a back surface 26. This microelectronic component assembly 10 is a conventional BOC package in which a back side 32 of a circuit board 30 is attached to the front surface 22 of the die 20 by adhesive members 35a and 35b. A passage 34 is formed through the entire thickness of the board 30 and permits access to the terminals 24 of the die 20 by a wire bonding machine or the like. The first adhesive member 35a extends adjacent one side of the passage 34 and the second adhesive member 35b extends along the opposite side of the passage 34.
The microelectronic component assembly 10 also includes a plurality of bond wires 40. A first set of bond wires 40a may extend from individual terminals 24 of the die 20 to a first set of bond pads 32a arranged on the front side 36 of the board 30 along a first side of the passage 34. Similarly, a series of second bond wires 40b may extend from other terminals 24 in the terminal array to a second set of bond pads 32b arranged on the front side 36 along the opposite side of the passage 34. Typically, these bond wires 40 are attached using wire-bonding machines that spool a length of wire through a capillary. A molten ball may be formed at a protruding end of the wire and the capillary may push this molten ball against one of the terminals 24, thereby attaching the terminal end 42 of the wire 40 to the die 20. The capillary moves laterally in a direction away from the bond pad 32 to which the wire 40 will be attached (referred to as the reverse motion of the capillary), then a further length of the wire will be spooled out and the board end 44 of the wire 40 will be attached to the bond pad 32. The reverse motion of the capillary is required to bend the wire into the desired shape to avoid undue stress at either the terminal end 42 or the board end 44. The need to move the capillary in the reverse direction to form the bend in the wire 40 requires significant clearance between the terminal end 42 and the inner surface of the passage 34, increasing the width W of the passage 34. The reverse motion also increases the length of each of the bond wires 40 and often requires an increased loop height L of the wire 40 outwardly from the front surface 22 of the die 20.
As noted above, most commercial microelectronic component assemblies are packaged in a mold compound 50. The mold compound 50 typically encapsulates the die 20, the adhesive members 35, the bond wires 40, and an inner portion of the board 30. A remainder of the board 30 extends laterally outwardly from the sides of the mold compound 50. In many conventional applications, the mold compound 50 is delivered using transfer molding processes in which a molten dielectric compound is delivered under pressure to a mold cavity having the desired shape. In conventional side gate molds, the mold compound will flow from one side of the cavity to the opposite side. As the front of the molten dielectric compound flows along the passage 34 under pressure, it will tend to deform the wires. This deformation, commonly referred to as “wire sweep,” can cause adjacent wires 40 to abut one another, creating an electrical short. Wire sweep may also cause one of the wires 40 to bridge two adjacent leads, creating an electrical short between the two leads. These problems become more pronounced as the wire pitch becomes smaller and as thinner wires 40 are used.
To protect the bond wires, a conventional BOC package is positioned in the mold cavity with the die oriented downwardly and the substrate oriented upwardly, i.e., generally in the orientation illustrated in FIG. 1. The mold compound 50 commonly flows longitudinally along the length of the passage 34 (in a direction perpendicular to the plane of the cross sectional view of FIG. 1) to create the lower portion of the mold compound 50, then flows in the opposite direction along the back side 32 of the board 30 to create the upper portion of the mold compound. In an attempt to keep the exposed portion of the substrate's front side 36 exposed, the back side 32 of the board 30 is typically supported by pins that extend upwardly from the bottom of the mold. The pressure of the mold compound 50 flowing along the passage 34, however, can force mold compound between the front face 36 if the board 30 and the surface of the mold cavity, leaving a flash coating of the mold compound on the front face 36. This flash coating must be removed before use if the contacts 37 on the front face 36 are used to electrically couple the microelectronic component assembly 10 to another component.